In the past there have been known navigation devices and navigation systems wherein a user is guided by using map data and road data to conduct a search for a route from a desired point of departure to a destination.
Examples of such navigation devices and navigation systems put into practice include car navigation systems for guiding an operator along a route, the system being installed in an automobile; communicable navigation systems wherein a portable phone is used as a navigation terminal, a route search request is sent to a route search server, the result is received, and route guidance is received; and the like.
The aforementioned car navigation systems uses a GPS (global positioning system), wherein GPS signals sent from a plurality of GPS satellites orbiting the globe are received by a GPS antenna; the satellite positions, time information, and the like included in the GPS signals are analyzed; and the position is specified. The number of the GPS satellites must be at least four. The independent positioning accuracy of a GPS is generally within 10 m, but this can be improved to 5 m or less by using a DGPS (differential GPS). Particularly, all models of portable phones referred to as third generation are tending to be equipped with positioning units that have presently only been installed in some portable phones, e.g., GPS receivers or the like which measure position by receiving signals from GPS (global positioning system) satellites.
A common navigation device, a route search device used in a communication navigation system, and a route search method are disclosed in Patent Reference 1 (Japanese Laid-open Patent Application No. 2001-165681) given below, for example. This navigation system is configured so that information on a point of departure and a destination is sent from a portable navigation terminal to a route search server, a road system and a traffic system are searched by the route search server for a route that matches the search conditions, and guidance is given. The search conditions include the means of moving from the point of departure to the destination, e.g., by foot, by automobile, a combination of by railroad and by foot, or the like; and this movement means is used as one search condition in the route search.
The route search server comprises roads (routes) of map data, wherein the positions of connecting points and turning points are nodes, the routes linking the nodes are links, and the cost information (distance and required time) of all the links is a database. The route search server can then guide the shortest route to the portable navigation terminal by referring to the database, sequentially searching for links beginning at the point of departure node and ending at the destination node, and creating a guiding route from the nodes and links having the minimal link cost information. A method known as label setting or the Dijkstra method is used as the method for this type of route search. A route search method which uses the Dijkstra method is also disclosed in Patent Reference 1.
Of all the routes from the point of departure to the destination obtained from a route search using road network data such as that described above, the route having the minimum cumulative route costs (distance or time) is determined to be the optimal guiding route, and guiding route data is created. In addition to the optimal route data, the guiding route data also includes map data and guidance data, and the guiding route data is read from a guiding data storing means and displayed on display means as necessary.
Generally, a guiding route and a mark indicating the current position of the navigation device are superposed on a map of a specified scale and a specified range including the current position of the navigation device, which has been positioned using the GPS receiver in the navigation device, and the current position mark is displayed so as to be in the center of the display image. In cases in which the navigation system has a voice guidance function, the guidance data is voice data, and guidance is given to the user by voice output.
The map data is sectioned into a mesh pattern with predetermined latitude and longitude ranges, and a map of a specific area is specified by mesh numbers. In a communicable navigation system, map data is distributed from the server to the terminal device, wherein the map data contains nine meshes adjacent to each other vertically, horizontally, and diagonally and centered around map data of a mesh including the current position of the terminal device. When the terminal device moves and the map data is insufficient, the terminal device sends a request to the server for a distribution of insufficient mesh map data. In an onboard navigation system, map data of the specified area is stored in advance on a hard disk device or the like.
When a map is displayed on the display means in this or another such navigation system, the road and its name are displayed in the vicinity of the corresponding road. For example, Patent Reference 2 (Japanese Laid-open Patent Application No. 2005-115174) given below discloses a map display device wherein the name of the corresponding road is displayed along the shape of the road.
The map display device disclosed in Patent Reference 2 is configured comprising a map display area determining unit, a road information acquiring unit, a map display data creating unit, and other components. The map display device determines an area for displaying the map, and the road information acquiring unit acquires turning point coordinate information and name information for the traffic route which is to be displayed in the determined display area from the map data sent from the map data distribution server. For every character or symbol constituting the character-symbol strings shown in the name information, the map display data creating unit determines a display position at a position that is located along the traffic route and that does not interfere with other characters and symbols, and the map display data creating unit creates map display data for displaying characters or symbols at the determined positions.
A typical electronic map used in a car navigation device or a map site on the Internet is generally configured from graphic components representing the terrain and character components for land names and the like. In the character components, road names are displayed in proximity to the roads, but in the case of raster maps, the display positions are often irrevocably determined. In vector maps, since the characters can also be depicted at arbitrary positions, the display positions are determined so that the road names can fit in the image.
For example, Patent Reference 3 (Japanese Laid-open Patent Application No. 9-101747) given below discloses an invention of a road map display device wherein specific positions in the display area of the road map (e.g., the area surrounding the display area) are set in advance as the road name display frame, the intersections between the road name display frame and the displayed roads are specified, and the road names are displayed at the intersections.    [Patent Reference 1]: Japanese Laid-open Patent Application No. 2001-165681 (FIGS. 1 and 2)    [Patent Reference 2]: Japanese Laid-open Patent Application No. 2005-115174 (FIGS. 1, 4, 6, and 16)    [Patent Reference 3]: Japanese Laid-open Patent Application No. 9-1011747 (FIGS. 2 and 4)